Prolonged capillary refill time — Push on your horse's gum. (vetmed.ucdavis.edu)
Dehydration in dogs should be assessed based on different common symptoms such as dullness, a lethargic appearance, increased heart rate but slower pulse rate, paleness and dryness over gums, dryness over the skin and delayed capillary refill time. (dog-health-handbook.com)
This is called slowed capillary refill time (CRT). (2ndchance.info)